Adsorption of Macrocyclic Cobalt Complex on a Glassy Carbon Electrode for the Electrocatalytic Reduction of O₂

It was found that the adsorption of a cobalt(III) complex with a macrocyclic ligand, C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc), was induced on a glassy carbon electrode by heavily oxidizing the electrode surface. Adsorption properties are discussed. The glassy carbon electrode with the adsorbed complex was employed to see the catalytic activities for the electro-reduction of O₂. In the presence of oxygen, reduction of (hmc)Co³⁺ showed two cathodic waves in cyclic voltammetry. Compared to the edge plane graphite electrode at which two cathodic waves were also observed in a previous study, catalytic reduction of O₂ occurred in the potential region of the first wave while it happened in the second wave region with the other electrode. A rotating disk electrode after the same treatment was employed to study the mechanism of the O₂ reduction and two-electron reduction of O₂ was observed. The difference from the previous results was explained by the different reactivity of the (hmc)CoOOH²⁺ intermediate, which is produced after the two-electron reduction of (hmc)Co³⁺ in the presence of O₂.